Torque sensors and axial-force sensors are varieties of mechanical quantity sensors. For example, there are many types of torque sensors for detecting torque acting on an axis of rotation. Magnetostrictive torque sensors having high precision and relatively simple configurations have become well-known in recent years, such as in JP-A-2001-133337 and JP-A-2004-309184.
The magnetostrictive torque sensors taught in JP-A-2001-133337 and JP-A-2004-309184 are provided to electric power steering apparatuses for use in vehicles and detect steering torque that is conveyed from a steering wheel to a torque-transmitting shaft (axis of rotation). A magnetostrictive film is formed on the outer perimeter surface of the torque-transmitting shaft. The change in magnetostriction that occurs in the magnetostrictive film in response to the steering torque is detected by the magnetostrictive torque sensor using an electrical coil and a magnetostriction-detection circuit, whereby the steering torque is detected.
Steering torque that is produced on the steering wheel is conveyed to the steered wheels via the torque-transmitting shaft, a rack-and-pinion mechanism, and a rack shaft. The torque-transmitting shaft is thus formed having a magnetostrictive film on the outer perimeter surface and a pinion (torque-transmitting part) of a rack-and-pinion mechanism on the end of the shaft.
A need has arisen for automotive vehicles that can be steered even when the engine has not been started. In this state, the steering torque for steering the wheels of the vehicle is larger than with normal steering. High steering torque is conveyed from the torque-transmitting shaft to the rack shaft via the rack-and-pinion mechanism. Rack-and-pinion mechanisms having a high mechanical strength are therefore needed. Specifically, various external forces caused by forces generated from a reaction with the road surface, as well as moderate external forces caused by driver steering act on the rack-and-pinion mechanism. The rack-and-pinion mechanism requires a mechanical strength that resists these external forces and allows the steering state to be maintained in those particular instances.
The pinion of the rack-and-pinion mechanism must adequately maintain the strength necessary to transmit steering torque exceeding normal levels to large loads. A variety of surface treatments are therefore frequently carried out on the pinion, such as carburization, induction hardening, other heat treatments, shot peening, and the like.
However, carrying out heat treatments on the pinion involves diffusing carbon components into the surface of the torque-transmitting shaft including the pinion. As a result, the surface of the torque-transmitting shaft is easily magnetized. Subjecting the pinion to shot peening and other surface-hardening treatments introduces residual compressive stress on the surface of the torque-transmitting shaft.
The magnetostrictive film formed on the outer perimeter surface of the torque-transmitting shaft is generally composed of a magnetostrictive plating of an Ni—Fe alloy film or the like. This kind of magnetostrictive plating is highly susceptible to the effects of magnetism from the torque-transmitting shaft and to the effects of strain on the torque-transmitting shaft.
Scope for improvement thus remains in regard to increasing the stability of the magnetostrictive properties of the magnetostrictive film when both a magnetostrictive film and a pinion (torque-transmitting part) are provided to the torque-transmitting shaft. Increasing the stability of the magnetostrictive properties is related to stabilizing the sensor signals of the magnetostrictive torque sensor.
This applies similarly to magnetostrictive axial-force sensors that have a shaft for transmitting axial force in order to convey axial forces to a load. Shafts for transmitting axial force are equivalent to the torque-transmitting shafts of magnetostrictive torque sensors. Scope for improvement remains in regard to increasing the stability of the magnetostrictive properties of the magnetostrictive film when both a magnetostrictive film and a part for transmitting axial force (the part that transmits axial force to a load) are provided to the shaft for transmitting axial force in a magnetostrictive axial-force sensor.
Accordingly, a technology is needed that can increase the stability of the magnetostrictive properties of the magnetostrictive film and that can provide both a magnetostrictive film and a torque-transmitting part to a torque-transmitting shaft, or both a magnetostrictive film and a part for transmitting axial force to a shaft for transmitting axial force, using processes optimized for each case.